1. Field of the Invention
The invention relates to acoustic surface wave processors and more particularly to an apparatus for performing convolution, correlation, time compression, time expansion, and time reversal.
2. Description of the Prior Art
Surface acoustic waves (SAW) have been employed for a multiplicity of signal processing applications, particularly as a wave convolver, such as that disclosed in Applicant's U.S. Pat. No. 3,833,867 entitled "Acoustic Surface Wave Convolver With Bidirectional Amplification" and in U.S. Pat. No. 4,016,513 both of which are assigned to the assignee of the present invention. These processors are essentially three port devices, two SAW input ports and an electrical output port. In these devices electrical signals, the convolution of which is desired, are coupled to transducers that launch surface acoustic waves towards an interaction region from either end thereof. At each point along the propagation path in the interaction region the product of the two counter propagating waves is taken due to a nonlinear interaction between the waves. This nonlinear interaction may be caused by the nonlinearity in the stiffness matrix that relates the stress and the strain in a surface wave material. Another cause is the electrical nonlinearity between piezoelectric fields produced by the counter traveling surface waves and the charge carriers in a semiconductor. The latter requires a piezoelectric media and a semiconducting media in close proximity to allow the electric fields produced in the piezoelectric media to extend into and interact within the semiconducting media. This is possible if the two media are the same, as for example, in CdS which is both semiconducting and piezoelectric, or if a film of piezoelectric material is deposited on a semiconductor material, or if the piezoelectric material and the semiconductor material, are pressed together but are separated by a very narrow air gap. A metal electrode positioned over the interaction region detects the product of the two signals at each point and takes a summation thereof, thus producing an electrical signal representative of the convolution integral for the two signal inputs. This output signal is not a propagating wave, it is a spatial average of an electrical signal which exists at the surface of the crystal.
In another device the two counter propagating signals are at different frequencies and the convolution signal is detected in the interaction region by an interdigital structure having a periodicity given by k.sub.3 =2.pi./D, where D is the distance between adjacent electrodes having the same polarity, k.sub.3 =k.sub.2 -k.sub.1 and w.sub.3 =w.sub.1 +w.sub.2. (w.sub.1, k.sub.1) and (w.sub.2, k.sub.2) are the frequencies and wave numbers of the two propagating waves launched from the oppositely positioned input transducers. The product signal is periodic and thus detected by a periodic interdigital transducer over the interaction region but is not a propagating surface wave. If w.sub.1 =w.sub.2 the convolver is degenerate and the detecting electrode structure is a solid metal electrode over the entire interaction region.
Convolvers and correlators of the prior art possess two input ports from which surface acoustic waves are launched and an electrical output port coupled to the interaction region of the two counter propagating waves. The present invention provides a device with four ports which may serve as input or output ports, three of which are utilized for any performable operation. An embodiment of the invention may be used for correlation, convolution, time expansion, time compression, and time reversal, with the signal resulting from the performed operation propagating as an surface acoustic wave from the interaction region to an output transducer for conversion to an electrical output signal.